Stress measuring device



Oct. 25, 1949. R. M. MAuNs STRESS MEASURING DEVICE 2 Sheet-Sheet 1 Filed March 11, 1946 ATTORNEY S QN um mv mv w WM u .11 d lfwf miv www mi@ MTww i mw bm ,im w 4 E @n xm mm mm w M mw@ m a MK k` R. M. MAINS STRESS MEASURING DEVICE Filed March l1, 1946 INVENTOR `@55W i5 xg@ @e ,OS

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Om @E @E g@ A ATTORNEY Patented Oct. 25, 1949 g 2,485,977 STRESS MEASURING DEVICE Robert M. Mains, Silver Spring, Md., assignor to the United States of America as represented by the Secretary of the Navy Application March 11, 1946, Serial No. 653,490

(Cl. 'i3-147) 7 Claims.

This invention relates to stress measuring means primarily designed for use in observing the performance of test bodies in a fluid current.

lOne object of the invention is to provide a means for measuring and determining the eiect of a fluidvcurrent on a test body such as a projectile, the measuring means being entirely electrical in operation and being adapted to give very exact readings of stresses of very small magnitude.

Another object of the invention is to provide a novel means for adapting SR-4 type bonded resistance wire strain gages for use in determining the behavior of a body such as a projectile l in a uid current.

Another object of thel invention is to provide a measuring device for' use in testing the behavior of abody Ysuch as a projectile in a fluid current, which has a minimum effect onA the" vfluid current used in the testing device and'l which is in turn not aiected by the uid current.

These and other objects of the invention will be apparent from the following specification and the accompanying drawings, in which:

Fig. 1 is a fragmentary diagrammatic plan view of a wind tunnel provided with testing means constructed in accordance with the present invention, showing a test body mounted therein, with partv oi the wind tunnel shell being broken away:

Fig. 2 is a longitudinal sectional View of the device shown in Fig. 1;

Fig. 3 is a longitudinal sectional view of the test body support showing the strain gages carried thereby, certain parts being shown in elevation and others broken'away;

Fig. 4 is a diagram of a circuit used in the testing apparatus;

Fig. V5 is a front elevational view of the operating panel of the apparatus;

Fig. 6 is a cross sectional and partly'diagrammatic view of the operating panel showing some of the elements contained in the circuit of Fig. 4.

As al stress sensitive element, "SR-4 type bonded resistance wire strain gages are utilized. lIhe sensory means employed in gages of this type consists of resistance wire attachable to the element in which the stresses'exist so that the resulting strains in the element deform the wire and thereby alter its electrical resistance. For example, awpiece of wire cemented firmly to the face of a flat piece of metal will partake ci the wire, will suffer changes in resistance which arefunctions of the strains in the flat piece of metal. Since strain is proportional to stress, the imposed stress may thus be determined. Gages of this type are manufactured commercially by the Baldwin Locomotive Works, Baldwin Southwark Division, and are described in the Baldwin Locomotive Works, Baldwin Southwark Division, Philadelphia, Pennsylvania, News Letter, volume 1, No. 2, for October 1944, entitled SRf-l News Letter and in the Bulletin No. 179 entitled The SR-4 bonded resistance wire strain gage, published by the same company. The details of such gages are also disclosed in Patent No. 2,292,549.

Referring to the drawings, the numeral I0 represents a wind tunnel in which is mounted a test body |I which is screwed onto the threaded end I'I of the supporting rod I8. The rod I8 is surrounded by a tubular shield I9 which pre- `vents the air currents from striking the rod |8 and setting up possible stresses which would give erroneous readings.

Therod I8 is supported within the shield I 9 as follows: At its rear end, the rod I8 has a shoulder 28 of reduced diameter upon which its a ring 2|. Beyond the shoulder 20, the rod is threaded to receive a corresponding threaded tube 22 which, when mounted on the end of rod I8, secures the ring 2| firmly in place.

An annular diaphragm 23 extends outwardly from the central ring 2| and carries on its outer periphery an outer ring 24. The outer ring 24 is clamped in a groove 25 in the shield |9.by means of a threaded tubular extension 26 of Said shield corresponding in length to the tube 22.

At the rear ends of the tubes 22 and 26, Similar joint constructions are provided to hold a pair of rings 2'I and 28 which support a diaphragm 29. A short tubular nut 30 holds the ring 2l against the end of tube 22 while a short tubular section 3| similarly holds the outer ring 28 in place. A tubular section 32 is then threaded onto the rear end of section 3| and is suitably supported in a rigid manner by a cross member |3 which extends transversely of the wind tunnel. The rod I8 is thus supported within the shield I9 by diaphragms 23 and 29V which permit axial movement of the rod IB as a whole while preventing any transverse movement thereof.

As the rod I8 has an over-al1 length of approximately 2 feet, it is bent somewhat by the transverse forces produced by the air current onl the body under test. The axial yield and 'of said bore in the rear end of rod i8.

transverse yield of the rod I8 are measured by means of the resistance wire strain gages as will now be described to determine wind resistance and yaw, respectively, of the test body.

Cemented or otherwise securely attached throughout its length to each face of each of the diaphragms 23 and 29 is a substantially circular rasistorvSS, 34, 35 and 35. A tot-alor four resistors are 'thus provided. They are of uniform diameter and arranged in pairs as shown, concentric with the rod I8 and located on opposite sides of the diaphragms.

Other wire strain gages comprising v'straight resistors are shown at 3l to-52, inclusiveVand are similarly secured. about the `peripheryof the rod I8 extending longitudinally thereof, at a-plurality of points along its length. These are arranged in sets of four disposed in spaced quadrature at several points along the length of the rod I8. 39 and 40 are on the 'upper surface of the rod I8, resistors 4I, 42, 43 land 44 are on the front v'surface ofthe rod, resistors 45, 546, 4l and 43 are on the lower surface of the rod, and resistors "49, 50, 5I and 52 are on the re'ar surface of the rod. Electrical leads 'from the various resistors are Apassed rearwardly along the outer surface ofthe rod IB and through openings I8a in the rod which communicate with the central bore of the tubular'elements 22 and 3u and the extension For purposes of clarity, all of "the leads are not shown, however, the lead 45a from the resistor 45 and lead 37a from gage 31 'are shown for illustrative purposes. Leads (not shown) from gage 36 are carried into the vcentral bore "through one 40i the openings 18a while leads (not shown)r from gages 34 and 35 pass into the bore through an openingfnot shown) in tube 22.

The resistors are well insulated by any suitable coating such as Bakelite, and are rmly secured "to, but insulated from their'supports by means of av-cement'of a suitable natur-e.

In order to measure strains v"of the rod I8, the wires from thevarious resistors are connected into electrical circuits, a sample of which is shown 'in' Fig. 4. A separate circuit is provided for veach of th-e opposite pairs of resistors. The typical circuit shown in Fig. 4 includes a Wheatstone bridge indicated by the diamond shaped figure ABCD with a 'direct current power source i 62 connected in one diagonal and a galvanometer 63 in the other diagonal. As shown, the sampl-e 'circuit includes the two resistors 3l and 45 and these resistors are shown in the arms B and A re- "spectively, of the bridge. `For purposes of ex-v ample assume that resistors 31 and 45 each have a resistance of approximately 120 ohms.

Another pair of resistors of approximatelythe Esame resistance, 53 and 54, are cemented tothe opposite faces 0f a metal strip 55 which is rigidly mounted at one end and at its other end is engaged bythe tip 58 of a micrometer 59 which is operableto deform the strip as shown by the dotted lines. .The resistors 53 and 54 comprise the arms D and C respectively of the Wheatstone lbridge Iso that Vthey balance the -resistors 3'! and 45 as the metal strip 55 is deformed by operation of the micrometer 59. When a balance is achieved as indicated by the galvanometer, the amount of linear deflection of the end of strip 55 is determined by reading the micrometer 59 which -is `calibrated in the desired units.

.In-order to compensate for unavoidable diierences in resistors, variable resistors 60 and. 6I are As shown in Fig. 3, the resistors 3?, `38, -1

sistors 31. and 45 are relatively small and could Abe easily masked by accidental lvariation in the bridge circuit, such as those which could be in- `troduced by the contact resistances of switches or the like, each bridge is made a permanently f wlred-and-'soldered circuit as far as its arms A, B, C and. D arel concerned. The only switching which is done in each of the Wheatstone circuits is theswitching in and out of the power supply and the galvanometer circuits.

V'As*bestsliown in Figs. 5 and 6, a suitable con- `trolspanel 65 is provided in a cabinet 65a which is mounted on a tab1e366. The table 66 also supports a power supply `62. y y y The panel includes a selector switch 64 for connecting th-e power source v62 and the g'alvanometerk 63 into the any particular VVi/hieat'storie circuit to `be measured. The micrometer heads are shown in Figs 5 and 6 by the'horizontal rows marked59 while the initial balance variable resistors 60 and 6I 'are also located in horizontal rowsY as indicated. A switch 69 Icontrols the power supply 62 and a pilot light 68 indicates when the poweris on. A-current reversingsw'itch 6'I .permits successive `measurements on both polarities of the bridge to eliminate errors Aas is conventional in 'Wheatstone bridge manipulai or other known stresses to the model and re- .-lthe.wind tunnel to the gal-vanomcter and power resistances to the next set to connect'the various pairs of resistances and Wheatstonebridges --in supply.

'Asi-the various pairs 'of resistors vare evenly 'spaced along the rod I 8, the strains in each set ishould' itheor'etically be llinear with the distance lfronifthe center of pressure on the device under test. By plotting the readings as ordinates against distances alongv the rod as abscissae, the

'center of pressure on the projectile or vother'test body will be indicated by the location 'at' which lthe resulting line on the graph, which Esllouldbe straight, crosses the axis of abscissae.

Any abnormal deviation of one of'the readingswill be evident immediately from the 4plots and the reading in question may be discarded as erroneous. In order to lincrease the reliability of the measurement for the center of pressure on the test body, a larger number of pairs of ref sistors may be used.

By noting the readings from the strain gages mounted on the diaphragms 23 and 29, the amount of axial stress of the rod I8 may be measured, giving an indication of the wind resistance of the body being tested.

Itis obvious that I have explained only a preferred embodiment of the invention and only a i'ew of the measurements which may be made by means of the invention. For example, if recording of the results of strain measurements is desired, the resistors 53 and 54 may be placed on the beam I8 adjacent the resistors 31 and 45 (see Figure 4). In this arrangement, the galvanometer 53 would be replaced by a suitable recording device, such as a record-ing potentiometer or a recording string galvanometer. This arrangement also permits better compensation for temperature changes experienced by the gages during operation. The invention is not limited to testing'projectiles but may be extended to aircraft or other test bodies and may be also used in other fluid currents besides air such as for testing ship hulls or other bodies in Water or other fluids.

I claim: v

1. Apparatus for examining the behavior of a test body in a iiuid current, comprising a test body supporting rod, a tube surrounding said rod and spaced therefrom, means for supporting said rod at one end within said tube, whereby a test body may be mounted upon the free end of said rod exposed to the on-coming uid flow, means for rigidly supporting said tube in said fluid current, and electrical means for measuring the strain of said rod caused by impingement of the fluid current on the test body.

2. Apparatus for examining the behavior of a test body in a fluid current, comprising a tubular member mounted in said uid current, a diaphragm mounted within said tube, a test body supporting rod carried within said tube by said diaphragm and having a free end projecting from the tube for mounting a test body thereon, resistance wire strain gages carried by said diaphragm and by said rod, and electrical measuring circuits connected with said gages for indicating distortion of said rod and said diaphragm caused by the impingement of the fluid current upon said test body.

3. Apparatus for examining the behavior of a test body in a fluid current, comprising a tubular member mounted in said uid current, a test body supporting rod suspended at one end within said tubular member and arranged to receive a test body thereon at the opposite end and position said body in contact with said fluid current, a pair of resistance wire strain gages attached to opposite surfaces of said rod, and an electrical measuring circuit connected with said gages for indicating distortion of said rod caused by impingement of the uid current upon said test body.

4. Apparatus for examining the behavior of test body in a fluid current, comprising a tubular member mounted in said fluid current, a test body supporting rod suspended at one end within said tubular member and arranged to receive a test body thereon at the opposite end and position said body in contact with said fluid current, a plurality of pairs of resistance wire strain gages spaced along said rod and attached thereto, the gages comprising each pair of gages being positioned apart on the periphery of said rod, and an electrical measuring circuit connectable to each pair of gages for determining the amount of distortion of said tube at the points of attachment of said gages, whereby phenomena associated with the impingement of fluid current on said body are ascertained.

5. An instrument for measuring forces exerted by a fluid current on a device immersed in said fluid, comprising an elongated exible member rigidly held at one end and at its other end supporting the device under test, a pair of electrical resistors located opposite each other on said member, whereby one becomes extended and the other compressed whenever the flexure of the member has a component in the plane of the resistors, and means for measuring the resistance changes of said resistors when thus stressed. I

6. An instrument as defined in claim 1, wherein there are two pairs of resistors, located in perpendicular planes.

'7. A measuring instrument for examining the effect of a fluid current on a test body, comprising a flexible arm supported rigidly at one end in line with the nuid flow and having means for carrying said body at its other end, a testing gage cooperating with said arm, and a faired spaced shield surrounding the exible arm to protect it from direct contact with the fluid current, said shield extending from the nxed end of the flexi- 1bletarm to a point just clear of the body under ROBERT M. MAINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,710,135 Zahm Apr. 23, 1929 2,213,982 Frey Sept. 10, 1940 2,252,464 Kearns, Jr., et al. Aug. 12, 1941 FOREIGN PATENTS Number Country Date 11,765 Great Britain 1902 490,483 Great Britain Aug. 16, 1937 

